Ion beam apparatus with separately replaceable elements

ABSTRACT

In an ion beam apparatus wherein ions are generated by the collision of electrons from a filamentary emitter with gas atoms in the vicinity of an electron-attracting grid structure, with subsequent extraction of the ions so generated from the vicinity of the grid structure and the focusing of these ions into a beam by an electrostatic lens system, a new way of mounting the ion extractor and the electrostatic lens system is disclosed which facilitates the removal and replacement of the filament and the grid structure. A configuration of the grid structure is also disclosed which optimizes the profile and the current density of the ion beam.

United States atent [1 1 Gerlach et a1.

[ Dec. 30, 1975 ION BEAM APPARATUS WITH SEPARATELY REPLACEABLE ELEMENTS[75] Inventors: Robert L. Gerlach, Los Altos;

George Rossini, Mountain View, both of Calif.

SUPPLY 3,369,148 2/1968 Hitchcock 250/423 3,547,074 12/ l 970 Hirschfeld.1 3,619,684 11/1971 Charlwood et al. 250/427 Primary Examiner-Archie R.Borchelt Assistant Examiner D. C. Nelms Attorney, Agent, or FirmStanleyZ. Cole; Leon F. Herbert; John J. Morrissey [57] ABSTRACT In an ion beamapparatus wherein ions are generated by the collision of electrons froma filamentary emitter with gas atoms in the vicinity of anelectron-attracting grid structure, with subsequent extraction of theions so generated from the vicinity of the grid structure and thefocusing of these ions into a beam by an electrostatic lens system, anew way of mounting the ion extractor and the electrostatic lens systemis disclosed which facilitates the removal and replacement of thefilament and the grid structure. A configuration of the grid structureis also disclosed which optimizes the profile and the current density ofthe ion beam.

18 Claims, 5 Drawing Figures US. Patent Dec. 30, 1975 Sheet10f23,930,163

FIG. I

FROM ARGON SUPPLY US. atent Dec. 30, 1975 Sheet20f2 3,930,163

W mm FW m m m 2 141m 12% m 0 nw 0 M23252; NE $5222 E 55x8 5% 22 E m 551325m 22 BEAM CROSS SECTION (mm) ION BEAM APPARATUS WITH SEPARATELYREPLACEABLE ELEMENTS BACKGROUND OF THE INVENTION l. Field of theInvention This invention is a further development in the art ofgenerating ion beams.

2. Description of the Prior Art Among the many applications involvingthe use of ion beams, the sputter cleaning or etching of surfaces is ofparticular importance. With an Auger surface analysis instrument, forexample, the chemical composition of the surface of a sample can benon-destructively analyzed by bombarding the surface with an electronbeam. However, it is frequently desirable to subject such a surface toion bombardment prior to electron bombardment in order to removeimpurities and contaminants from the surface. If it is desired toconduct a continuing Auger analysis of successive layers within thesample, it becomes necessary to remove each successive layer by ionbombardment after having been analyzed so that fresh layers can beexposed to the electron beam for analysis.

In ion beam bombardment apparatuses known to the prior art, the ion beamsource and the target to be bombarded by the ion beam were mountedwithin an evacuable envelope, and an atmosphere of ionizable gas such asargon was maintained in the envelope at a pressure in the range from 10to ltorr. The ion beam source comprised an electron emitter, anelectron-attracting grid structure, an ion extractor and anelectrostatic lens system. The electron emitter was typically afilamentary cathode. The grid structure was a helically wound cage-likewire structure typically made from 0.005-inch diameter tungsten wirewound to a pitch of turns per inch. The helical configuration of thegrid structure for prior art apparatuses was apparently a carry-overfrom the design of the grid structure for vacuum-ionization gauges suchas the gauge described in U.S. Pat. No. 3,435,334, issued on Mar. 1969and assigned to Varian Associates. Electrons from the filament wereattracted to the vicinity of the grid structure, whereupon inelasticcollisions of these electrons with gas atoms in the vicinity of the gridstructure would produce ions. The ion extractor was typically acylindrical screen surrounding both the filament and the grid structureand was maintained at suitable electrical potential to extract the ionsaway from the vicinity of the grid structure and to accelerate the ionstoward the region of influence of the electrostatic lens system. Theelectrostatic lens system served to focus the extracted ions into a beamand to accelerate the ions of the beam toward the target. The principlesof electrostatic lens systems are well-known, and are treated in textssuch as Electron Optics by B. Paszkowski, Iliffe Books Ltd., London,1968 and Electron Optics by P. Grivet, Pergamon Press, London, 1965.

In the ion beam apparatuses of the prior art, the electron-emittingfilament, the electron-attracting grid structure and the ion extractorwere all mounted as a single assembly on a vacuum feed-through member,which provided electrical connections to the outside of the evacuableenvelope. Sometimes the various components of the electrostatic lenssystem were also mounted as parts of the same assembly. To replace afilament or a grid structure, it was necessary to demount the ionextractor from its position surrounding the filament and the gridstructure. Sometimes it was also necessary to demount the electrostaticlens system in order to replace the filament or the grid structure. Thegrid structure of the prior art was essentially a helical wire havingrod-like supporting members at tached thereto as by welding. The gridstructure supporting members were affixed to the vacuum feedthroughmember by barrel connectors or by welding. Thus, to replace a gridstructure in the prior art required the unscrewing of a number of barrelconnector screws to remove the old grid structure and thesubsequentrealignment of the new grid structure while reinserting andtightening the barrel connector screws, or else it required the breakingand subsequent reforming of a number of welds.

SUMMARY OF THE INVENTION The present invention provides an improvedmounting arrangement for the filament, grid structure, ion extractor andelectrostatic lens system of an ion beam apparatus. In addition, a newconfiguration for the grid structure of the ion beam apparatus isprovided which optimizes the beam profile and the current density of theion beam.

In particular, the electron-emitting filament and theelectron-attracting grid structure are mounted on one assembly, whilethe ion extractor and the electrostatic lens system components aremounted on a different assembly. The filament-grid assembly is joined tothe extractor-lens assembly by having a mating portion of one assemblyplug into a corresponding mating portion of the other assembly. However,the two assemblies are readily separable, thereby providing easy accessto the filament and grid structure without having to demount the ionextractor and the lens system components from their assembly. The gridstructure is demountably attached to the filament-grid assembly by asingle screw, rather than by a number of barrel connectors as in theprior art. Thus, with the mounting arrangement of the present invention,replacement of the grid structure is much easier than was the case withprior art apparatuses. With the present invention, the extractor-lensassembly as a whole can be unplugged from the filament-grid assembly,thereby providing access to the filament and the grid without having todemount the ion extractor and the components of the electrostatic lenssystem. The grid structure itself can be removed from the filament-gridassembly by removing a single screw, and a replacement grid structurecan be substituted in proper alignment by reinserting the single screw.The tedious task inherent in the prior art mounting technique ofadjusting a number of barrel connectors to achieve proper alignment ofthe grid structure is thereby obviated.

In addition, the grid structure of this invention is made from arefractory metal into a fine mesh, rather than into a helical structureas in the prior art. The mesh configuration of this invention providesseveral significant advantages over the helical configuration of theprior art. For example, it has been found that for a givenfilament-to-grid spacing, the space-charge limited current occurs at alower voltage with the fine mesh grid structure than with the helicalgrid structure. This means that electrons are drawn off the filamentmore efficiently with the fine mesh grid structure. This greaterefficiency is attributable to the greater electric field strength thatcan be obtained between the filament and the fine mesh grid of thisinvention than was possiblebetween the filament and the helical grid ofthe prior art. It has also been found that for a given filament-to-gridelectron emission current, the resulting ion beam current density issignificantly higher with the fine mesh grid of this invention than withthe helical grid of the prior art. This increased ion beam currentdensity is attributable to the fact that a more uniform electric fielddistribution can be maintained in the region of the fine mesh gridconfiguration than inside the helical grid configuration of the priorart, so that a more uniform electron density can be achieved in thevicinity of the fine mesh grid configuration of this invention. Inaddition, it has been found that the. beam profile of the ion beamgenerated by an apparatus hav ing a fine mesh grid configuration issuperior to the profile of an ion beam generated by a prior artapparatus. Specifically, the ion beam profile generated by an apparatusaccording to this invention is narrower and more uniform than the ionbeam profile obtainable in the prior art. This narrower and more uniformion beam profile is likewise attributable to the fact that the electricfield distribution and the electron density within the fine mesh gridstructure are more uniform than in the helical type of grid structureused in the prior art.

BRIEFDESCRIPTION OF THE DRAWING FIG. 1 shows a longitudinalcross-sectional view of an ion beam apparatus which embodies the presentinventron.

FIG. 2,shows an alternative configuration for the grid structure of thepresent invention.

FIG. 3 shows a fragmentary longitudinal cross-sectional view of analternative embodiment of the ion extractor of. an ion beam apparatusaccording to the present invention.

FIG. 4 shows curves indicative of the ion beam profile of prior artapparatuses.

FIG. 5 shows curves indicative of the ion beam profile of an apparatusaccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 shows an ion beam apparatuswherein an ion beam source and a means for holding a sample 21 forbombardment by the ion beam 11 are mounted within a chamber which issurrounded by a vacuumtight enclosure 22. The sample holding means 20may be an electrically grounded sample holder of any suitable mechanicaldesign such as, for example, the carousel-type of sample holderdisclosed in copending US. Pat. application Ser. No. 453,749, filed onMar. 22, 1974 and assigned to Varian Associates. The chamber 25 isevacuable through port 23 in wall 22 by a suitable vacuum pumping means,and an inert ionizable gas such as argon can be introduced into theevacuated chamber 25 through port 24 in wall 22.

The ion beam source 10 comprises a filament-grid assembly designated bythe reference number /40 and an extractor-lens assembly designated bythe reference number 50/60. The principal components of thefilament-grid assembly 30/40 are an electron-emitting filament 30 and afine mesh grid structure 40. The principal components of theextractor-lens assembly 50/60 are an ion extractor 50 and anelectrostatic lens system 60.

The filament 30 is preferably made of 0.007-inch diameter non-sagtungsten wire of the type sold by General Electric Company under thecatalog designation 2l8-tungsten. The filament 30 is formed into forexample a hair-pin configuration, and the prongs thereof are demountablyattached by barrel connectors 31 and 32 to rigid electrical leads 33 and34, respectively, which extend through an insulating ceramic plate 35 toelectrical contact pins (not shown) outside the vacuum enclosure 22. Theelectrical contact pins provide connection with an electrical circuitfor resistively heating the filament 30 to a temperature at whichelectrons will be emitted therefrom, and for maintaining the filament ata desired electrical potential as of 2.8 kilovolts.

The grid structure 40 comprises a generally cylindrically shaped finemesh 41, which is made from a refractory metal such as tungsten,iridium, tantalum or molybdenum. The mesh 41 may be a net-like screen ofinterwoven wires as shown in FIG. 1, or alternatively may be formed froma perforated sheet of refractory metal as shown in FIG. 2. Theperforations in the metal sheet may be circular as illustrated in FIG.2, or of any other shape such as square or rectangular. The mesh 41 ischaracterized as a fine mesh, the measured fineness being in the rangefrom 20-mesh to IOO-mesh. Thus, for either the net-like screenembodiment or the perforated sheet embodiment, the mesh 41 is equivalentin fineness to a screen as of for example 0.00l-inch diameter wiresevenly spaced apart at a closeness in the range from 20 to 100 per inch.Preferably, a IOO-mesh fineness is used. The mesh 41 is inherentlynon-rigid. Therefore, in the case of the net-like screen embodiment, theends of the non-rigid mesh cylinder are affixed as by welding to metalrings 42 and 43, one at either end of the cylinder. Two or more rigidsupport members 44 extending the length of the cylinder are joined tothe ring 42 and to the ring 43 so as to provide a rigid frame formaintaining the non-rigid mesh 41 in a rigid cylindrical configuration.In the case of the net-like screen embodiment shown in FIG. 1, one endof each of several metal stilt-like supports 45 is affixed as by weldingto ring 43. In the case of the perforated sheet embodiment shown in FIG.2, a rigid cylindrical configuration can be maintained for the mesh 41by extending each of the metal stilt-like supports 45 the length of thecylinder. In either case, the other end of each of the supportingmembers 45 is affixed as by welding to a cup-like metal base member 46.The base member 46 is demountably attached by means of a single screw 47to a metal plate 48, which is affixed to ceramic plate 35 by screws 36and 37. Electrical lead 38 forms an electrical contact with bolt 37 andextends to an electrical contact pin (not shown) extending outside thevacuum enclosure 22. The electrical contact pin provides connection withan electrical circuit for maintaining the grid structure 40 at a desiredelectrical potential as of 3.0 kilovolts.

The ceramic plate 35 is bolted by a number of bolts, which arerepresented in FIG. 1 by bolt 70, onto one side of a flanged annularmetal fitting 39. The ceramic plate 35 covers the opening on one side ofthe fitting 39. The electrical leads 33 and 34 which pass through theceramic plate 35, and the electrical lead 38 which is connected to theelectrically conducting bolt 37 which passes through the ceramic plate35, all extend into the opening through the fitting 39. Other electricalleads 84, and 86, whose function will be discussed hereinafter, alsoextend into the opening through the fitting 39. A ceramic plug 71 fitssnugly into the opening at the other side of the fitting 39. Theelectrical leads 33, 34, 38, 84, 85 and 86 all pass via the openingthrough fitting 39, and thence through the ceramic plug 71, toelectrical contact pins (not shown) which extend outside the vacuumenvelope 22. The flanged annular fitting 39 is seated in an outer flange72. Vaccum-tight seals between the ceramic plug 71 and the fitting 39,and between the fitting 39 and the outer flange 72, are effected bymeans (not shown) which are well-known to the vacuum art. Outer flange72 is bolted by bolts 74 and 75 onto a mating flange 73 which is formedintegrally with the enclosure wall 22 around an opening 76 which islarge enough to permit the ion beam source to project into the chamber25. Vacuum tightness of the mating of the flanges 72 and 73 is providedby the metal sealing gasket 77, as described more fully in US. Pat. No.3,208,758, issued on Sept. 28, 1965 and assigned to Varian Associates.

The cylindrical axis of the grid structure 40 is aligned substantiallyparallel to the prongs of the hair-pin shaped filament 30. It is afeature of this invention that the grid structure 40 can easily bedemounted, and a new grid structure can be substituted in its place inproper alignment with the prongs of the filament 30, simply by removingand subsequently reinserting the the single screw 47. In the gridstructure mounting technique ofthe prior art, on the other hand, anumber of supporting members affixed to a helical type of grid structurewere joined by barrel connectors to a corresponding number of supportmembers affixed to an insulating base. It was a tedious process with themounting technique of the prior art to adjust the barrel connectors sothat the grid structure could be properly aligned. With the mountingtechnique of the present invention, proper alignment of the gridstructure can be attained by tightening the single screw 47.

Another difficulty experienced with prior art mounting techniques wasthat in prior art apparatuses the ion extractor and sometimes also thecomponents of the electrostatic lens system were mounted as parts of thesame assembly to which the grid structure and the filament were mounted.Thus, in the prior art it was necessary to demount the ion extractor andperhaps also the components of the electrostatic lens system in order tohave access to the grid structure or the filament. In the presentinvention, on the other hand, the ion extractor 50 and the electrostaticlens system 60 are mounted as parts of an assembly 50/60 which is easilyseparable from the assembly 30/40 which comprises the filament 30 andthe grid structure 40.

The ion extractor 50 comprises a generally cylindrical net-like screen51 which surrounds the mesh 41 of the grid structure 40 and the greaterportion of the filament 30. The cylindrical axis of the extractor 50 isaligned substantially parallel to the cylindrical axis of the gridstructure 40 and to the prongs of the filament 30. In the preferredembodiment, the netlike screen 51 is made of 0.003-inch diameter metalwire as of stainless steel of I00 mesh. A screen made to thesespecifications will remain rigid without crumpling at the temperaturesusually encountered. The screen 51 is affixed as by welding to a flangedmetal annular member 52 which covers the end of the extractor 50adjacent the electrostatic lens system 60, except for an annular opening55 therein which is concentric with the grid structure 40 but of adiameter which is smaller by approximately percent. Ions extracted fromthe vicinity of the grid structure are accelerated through the opening55 toward the electrostatic lens system 60. The

6 annular member 52 provides the means whereby the extractor 50 ismounted within the ion beam source 10. A metallic cylindrical sleeve 53having an electrically insulating ion extractor support member affixedlydisposed therein, such as a ceramic support 54, surrounds but isseparable from the filament-grid assembly 30/40. A flanged portion ofthe metal annular member 52 is received in and is fixedly attached to amating portion of the ceramic support 54, whereby the ion extractor 50is mechanically mounted on the sleeve 53 but is electrically insulatedtherefrom. It is anticipated that the net-like screen 51 described abovecould be replaced by a cylindrical structure made from a sheet ofperforated metal of IOO-mesh. The ion extractor 50 is maintained at anelectrical potential as of 2.7 kilovolts to attract positive ions awayfrom the vicinity of the grid structure 40, which is maintained at ahigher electrical potential as of 3.0 kilovolts. The ion extractor 50thus serves to isolate the filament 30 and the grid structure 40electrostatically from the metal sleeve 53, and to accelerate the ionstoward the electrostatic lens system 60. The operation of the ionextractor 50 is well-known and is explained in more detail in texts such3 as Ion Bombardment of Solids by G. Carter and J. S

Colligon American Elsevier Publishing Company, Ne w York, 1968,particularly at pages 423 et seq. Aninnovative alternative embodimentfor the ion extractor 50 is shown in FIG. 3. In particular, theinnovation provides for a screen 56 of IOO-mesh, 0.001-inch diametertungsten wire to be mounted on a sleeve 57 projecting from the annularcover member 52 into the nearest lens element 61 of the electrostaticlens system 60. It has been found that the apertured end screen 56 incombination with the projecting sleeve 57 causes a sharper focusing ofthe ion beam at lower energies than is possible without the end screen56 and the projecting sleeve 57. In the embodiment shown in FIG. 3, theprojecting sleeve 57 is formed integrally with the cover member 52 andextends into the interior of a cylindrical electrostatic lens element61. The apertured end screen 56 is affixed as by welding to the end ofthe sleeve 57 and covers the opening 55 in the cover member 52. Ionsgenerated in the vicinity of the grid structure 40 pass through thescreen 56'into the electrostatic lens system 60 wherein they are formedinto an ion beam 11. An electrical lead 58, which terminates in anelectrical contact plug 59, is affixed to the ion extractor 50 andprovides the means for maintainingthe ion extractor 50 at the desiredelectrical potential. The plug 59 is insertable into an electricallyconductive female contact (not shown) in an insulator which is fixedlyattached to the metal sleeve 53. An electrical path is provided fromplug 59 through the insulator 90 to a male contact (not shown) on theopposite side of the insulator- 90. This male contact of the insulator90 is insertable into a metallic female receptor (not shown) in theceramic plate 35, which provides an electrical path through the ceramicplate 35 to an electrical lead 86. The electrical lead 86 passes via theopening in fitting 39 through the ceramic plug 71 to an electricalcontact pin (not shown) which extends outside the vacuum envelope 22.The ion extractor 50 is maintained at its proper electrical potential bymeans of this external electrical contact pin.

The electrostatic lens system 60 comprises, typically, a number ofcylindrically configured lens elements, as indicated by referencenumbers 61, 62 and 63, which form the ions into an ion beam and whichaccelerate the ions of the beam toward the target 21. The lens elements61, 62 and 63 are mounted on the sleeve 53in a manner similar to the wayin which the ion extractor 50 is mounted on the sleeve 53. As shown inFIG. 1, electrically insulating support members 64, 65 and 66 areaffixedly disposed within the metal sleeve 53 and serve as mechanicalsupports for the lens elements 61, 62 and 63, respectively. The lenselements 61, 62 and 63 are maintained at their required electricalpotentials by electrical leads 67, 68 and 69, respectively. In theelectrostatic lens system configuration shown in FIG. 1, lens elements61 and 63 are maintained at the same electrical potential, so thatelectrical lead 69 can be connected to electrical lead 67 in the spacewithin the sleeve 53. The electrical leads 67 and 68 then pass throughthe region within the sleeve 53 externally of the ion extractor 50, andterminate in electrical contact plugs 87 and 88, respectively. The plugs87 and 88 are insertable into electrically conductive female contacts 92and 93, respectively, in the insulator 90. An electrical path isprovided from plug 87 through the insulator 90 to a male contact 94, andan electrical path is provided from plug 88 through the insulator 90 toa male contact 95, both contacts 94 and 95 being on the opposite side ofthe insulator 90 from the female contacts 92 and 93. The male contacts94 and 95 are insertable into metallic female receptors 82 and 83,respectively, in the ceramic plate 35. The ends of the receptors 82 and83 facing the region between the ceramic plate 35 and the ceramic plug71 are in electrical contact with electrical leads 84 and 85,respectively. The electrical leads 84 and 85 pass via the opening infitting 39 through the ceramic plug 71 to electrical contact pins (notshown) which extend outside the vacuum envelope. The lens elements ofthe electrostatic lens system 60 are maintained at their properelectrical potentials by means of these external electrical contactpins.

The metal sleeve 53, together with the electrostatic lens systemsupporting members 64. 65 and 66 and the lens elements 61, 62 and 63 sosupported, as well as the electrical leads 67, 68 and 69 with theirterminating plugs 87 and 88, and also together with the ion extractor 50and its supporting member 54 and the electrical lead 58 and itsterminating plug 59, along with the plug-receiving insulator 90,collectively comprise the extractor-lens assembly 50/60. It can beappreciated from FIG. 1 that the entire extractor-lens assembly 50/60can be mounted in proper position with respect to the filament-gridassembly 30/40 by placing the sleeve 53 over the ceramic plate 35 suchthat the filament 30 and the grid structure 40 are received into thecylindrical interior of ion extractor 50, and such that the malecontacts 94 and 95 for the electrostatic lens system 60 and the malecontact (not shown) for the ion extractor 50 are inserted intoelectrically conductive frictional contact with the corresponding femalereceptors 82, 82 and the other contact not shown in the ceramic plate35. The sleeve 53 fits snugly over the ceramic plate 35 and also over aflanged portion of the fitting 39. The sleeve 53 is firmly butdemountably attached to the fitting 39 by one or more screws representedby reference number 99 which pass through aligned threaded holes in boththe sleeve 53 and the fitting 39. To demount the extractor-lens assembly50/60, it is only necessary to remove screws 99 and to lift the sleeve53 away from the ceramic plate 35, thereby easily exposing the filament30 and the grid structure 40.

The advantages of an ion beam apparatus according to this invention overprior art apparatuses are clearly indicated by the curves shown in FIGS.4 and 5. The curves in FIG. 4 represent plots of the ion beam currentdensity, measured in microamperes per square centimeter, versus thecross-sectional width of the ion beam, measured in millimeters, for ionbeams of two different energy levels, namely 1000 electron volts and2,000 electron volts, generated by an apparatus of the prior art. Thecurves in FIG. 5 represent the same parameters for an apparatusaccording to the present invention. It can be seen by comparing thecurves of FIGS. 4 and 5 that for a given ion beam energy, an apparatusaccording to the present invention provides a beam which is narrower andmore symmetrical than could be obtained with prior art apparatuses.

Variations in particular details of the construction of an ion beamapparatus according to this invention may be made without departing fromthe scope of the invention as described herein. Accordingly, theinvention is limited only by the following claims.

What is claimed is:

1. An ion beam source comprising an electron emitter, a grid structuremaintainable at an electrical potential to attract electrons from saidemitter toward the vicinity of said grid structure for collision withionizable gas atoms in the vicinity of said grid structure therebyproviding ions, said grid structure comprising a refractory metal mesh,means for extracting said ions from the vicinity of said grid structure,and means for forming said ions into an ion beam, said electron emitterand said grid structure being mounted on a first assembly member andforming therewith a first assembly unit, said ion extracting means andsaid ion beam forming means being mounted on a second assembly memberand forming therewith a second assembly unit, said second assemblymember being demountably secured relative to said first assembly member,and said electron emitter being demountably secured to said firstassembly member, whereby said second assembly unit is insertable andremovable separately from said first assembly unit and said electronemitter is insertable and removable as an individual unit.

2. The ion beam source of claim 1 wherein said refractory metal mesh ismade from a metal chosen from the group consisting of tungsten, iridium,tantalum and molybdenum.

3. The ion beam source of claim 1 wherein said mesh comprises a net ofinterwoven wires.

4. The ion beam source of claim 1 wherein said mesh comprises aperforated metal sheet.

5. The ion beam source of claim 1 wherein said first assembly membercomprises a ceramic plate and said second assembly member comprises ametallic sleeve, said metallic sleeve being dimensioned to fit snuglyover said ceramic plate.

6. The ion beam source of claim 1 wherein said electron emitter is afilamentary electrode.

7. The ion beam source of claim 1 wherein said mesh is formed into agenerally cylindrical configuration.

8. The ion beam source of claim 1 wherein said ion extracting means hasa generally cylindrical configura tion and surrounds the greaterportions of said electron emitter and said grid structure.

9. The ion beam source of claim 8 wherein one end of said cylindricallyconfigured ion extracting means is disposed adjacent said ion beamforming means, and wherein a sleeve member projects from said ion ex- 9tracting means toward said ion beam forming means, and the end of saidsleeve member toward said ion beam forming means is covered with anapertured screen.

10. The ion beam source of claim 1 wherein said ion beam forming meanscomprises an electrostatic lens system.

11. An ion beam bombardment apparatus for bombarding a sample, saidapparatus comprising an envelope for containing an ionizable gas, meansfor holding said sample within said envelope, an electron emittermounted within said envelope, a grid structure mounted within saidenvelope, said grid structure being maintainable at an electricalpotential to attract electrons toward the vicinity of said gridstructure whereby collisions of said electrons with atoms of said gasproduce ions of said gas, said grid structure comprising a refractorymetal mesh, means for extracting said ions from the vicinity of saidgrid structure, and means for forming said ions into an ion beam,whereby said samples may be subjected to ion sputter-etching, saidelectron-emitter and said grid structure being mounted on a firstassembly member, and forming therewith a first assembly unit, said ionextracting means and said ion beam forming means being mounted on asecond assembly member and forming therewith a second assembly unit,said second assembly member being demountably secured relative to saidfirst assembly member, and said electron emitter being demountablysecured to said first assembly member, whereby said second assembly unitis insertable and removable separately from said first assembly unit andsaid electron emitter is insertable and removable as an individual unit.

12. The apparatus of claim 11 wherein said ionizable gas is argon.

13. The apparatus of claim 11 wherein said refractory metal meshcomprises a net of interwoven wires.

14. The apparatus of claim 13 wherein said refractory metal meshcomprises a perforated metal sheet.

15. The apparatus of claim 11 wherein said envelope comprises a closurefitting for hermetically sealing an opening in the wall of saidenvelope, a plate mounted on the inner side of said fitting and formingsaid first assembly member, said ion extracting means and said ion beamforming means being mounted on the inward portion of a cylinderprojecting into said envelope, the outward portion of said cylinderbeing positioned around said electron emitter and grid structure, andsaid cylinder forms said second assembly member.

16. The apparatus of claim 15 wherein the outward end portion of saidcylinder fits snugly around said plate.

17. The apparatus of claim 16 wherein said fitting includes an inwardlyextending flange on which said said flange.

1. An ion beam source comprising an electron emitter, a grid structuremaintainable at an electrical potential to attract electrons from saidemitter toward the vicinity of said grid structure for collision withionizable gas atoms in the vicinity of said grid structure therebyproviding ions, said grid structure comprising a refractory metal mesh,means for extracting said ions from the vicinity of said grid structure,and meaNs for forming said ions into an ion beam, said electron emitterand said grid structure being mounted on a first assembly member andforming therewith a first assembly unit, said ion extracting means andsaid ion beam forming means being mounted on a second assembly memberand forming therewith a second assembly unit, said second assemblymember being demountably secured relative to said first assembly member,and said electron emitter being demountably secured to said firstassembly member, whereby said second assembly unit is insertable andremovable separately from said first assembly unit and said electronemitter is insertable and removable as an individual unit.
 2. The ionbeam source of claim 1 wherein said refractory metal mesh is made from ametal chosen from the group consisting of tungsten, iridium, tantalumand molybdenum.
 3. The ion beam source of claim 1 wherein said meshcomprises a net of interwoven wires.
 4. The ion beam source of claim 1wherein said mesh comprises a perforated metal sheet.
 5. The ion beamsource of claim 1 wherein said first assembly member comprises a ceramicplate and said second assembly member comprises a metallic sleeve, saidmetallic sleeve being dimensioned to fit snugly over said ceramic plate.6. The ion beam source of claim 1 wherein said electron emitter is afilamentary electrode.
 7. The ion beam source of claim 1 wherein saidmesh is formed into a generally cylindrical configuration.
 8. The ionbeam source of claim 1 wherein said ion extracting means has a generallycylindrical configuration and surrounds the greater portions of saidelectron emitter and said grid structure.
 9. The ion beam source ofclaim 8 wherein one end of said cylindrically configured ion extractingmeans is disposed adjacent said ion beam forming means, and wherein asleeve member projects from said ion extracting means toward said ionbeam forming means, and the end of said sleeve member toward said ionbeam forming means is covered with an apertured screen.
 10. The ion beamsource of claim 1 wherein said ion beam forming means comprises anelectrostatic lens system.
 11. An ion beam bombardment apparatus forbombarding a sample, said apparatus comprising an envelope forcontaining an ionizable gas, means for holding said sample within saidenvelope, an electron emitter mounted within said envelope, a gridstructure mounted within said envelope, said grid structure beingmaintainable at an electrical potential to attract electrons toward thevicinity of said grid structure whereby collisions of said electronswith atoms of said gas produce ions of said gas, said grid structurecomprising a refractory metal mesh, means for extracting said ions fromthe vicinity of said grid structure, and means for forming said ionsinto an ion beam, whereby said samples may be subjected to ionsputter-etching, said electron-emitter and said grid structure beingmounted on a first assembly member, and forming therewith a firstassembly unit, said ion extracting means and said ion beam forming meansbeing mounted on a second assembly member and forming therewith a secondassembly unit, said second assembly member being demountably securedrelative to said first assembly member, and said electron emitter beingdemountably secured to said first assembly member, whereby said secondassembly unit is insertable and removable separately from said firstassembly unit and said electron emitter is insertable and removable asan individual unit.
 12. The apparatus of claim 11 wherein said ionizablegas is argon.
 13. The apparatus of claim 11 wherein said refractorymetal mesh comprises a net of interwoven wires.
 14. The apparatus ofclaim 13 wherein said refractory metal mesh comprises a perforated metalsheet.
 15. The apparatus of claim 11 wherein said envelope comprises aclosure fitting for hermetically sealing an opening in the wall of saidenvelope, a plate mounted on the inner side of said fitting and formingsaid first aSsembly member, said ion extracting means and said ion beamforming means being mounted on the inward portion of a cylinderprojecting into said envelope, the outward portion of said cylinderbeing positioned around said electron emitter and grid structure, andsaid cylinder forms said second assembly member.
 16. The apparatus ofclaim 15 wherein the outward end portion of said cylinder fits snuglyaround said plate.
 17. The apparatus of claim 16 wherein said fittingincludes an inwardly extending flange on which said plate is mounted,and the outward end of said cylinder fits snugly around said flange. 18.The apparatus of claim 17 further comprising screw means securing saidplate to said flange and screw means independently securing saidcylinder to said flange.